Cholesterol is a chemical compound with a rather bad image. High cholesterol levels are commonly associated to malnutrition and a poor health status leading to cardiovascular diseases like heart attacks or strokes. This is due to cholesterol’s tendency to form solid deposits in arteries causing blockages and arteriosclerosis. The insight, that cholesterol in particular is found in such deposits is surprisingly old: it was published already in 1911 by the laboratory of Adolf Windaus, who later received the 1928 Nobel Prize in Chemistry for elucidating the structures of cholesterol and several related compounds. Large epidemiological studies, published during the middle of the 20th century, then unambiguously confirmed that high blood cholesterol levels are in fact a major risk factor for arteriosclerosis.Today we know that, despite its bad public image, cholesterol is essential to all mammalian life. Our cells require it to maintain their plasma membranes and important biomolecules like vitamin D or hormones such as testosterone and oestrogen are biosynthesized from it. So how can a molecule so important as cholesterol become such a menace? This question could only be properly understood on the basis of the work of Konrad Bloch and Feodor Lynen. The two German-born scientists had shown how our bodies metabolize fats and cholesterol and regulate their levels. They eventually received the 1964 Nobel Prize in Physiology or Medicine for this achievement. In the present lecture, Feodor Lynen gives a review of this work and further research on cholesterol done after the 1964 award. He points out that the cholesterol required by our body is obtained both from food and from a complicated biosynthetic pathway comprising 37 reaction steps. The latter mainly takes place in the liver. In a healthy human, the biosynthesis of cholesterol is adjusted according to the intake via food. If more cholesterol is consumed, less cholesterol is made in the liver and vice versa. In the liver, cholesterol is furthermore biodegraded to bile acids, major constituents of bile (gall), which are excreted in the frame of digestion, thus further lowering blood cholesterol. However, these regulation mechanisms have limitations and can be affected by diseases or individual predisposition. In this context, Lynen mentions that saturated fats increase cholesterol blood levels more than unsaturated fats do. Since meat products are rich in saturated fats, a good (and rather popular) way of reducing the risk of cardiovascular diseases is a diet rich in vegetables, Lynen says.He then goes on to discuss the biosynthesis and regulation of cholesterol in more chemical detail. In his 1975 Lindau lecture, given three years after the present one, he should discuss this particular issue again, incorporating results obtained in the meantime. David Siegel